Add payload to the message object's existing payload. If, prior
to calling this method, the object's payload was None
(i.e. never before set), then after this method is called, the payload
will be the argument payload.

If the object's payload was already a list
(i.e. is_multipart() returns 1), then payload is
appended to the end of the existing payload list.

For any other type of existing payload, add_payload() will
transform the new payload into a list consisting of the old payload
and payload, but only if the document is already a MIME
multipart document. This condition is satisfied if the message's
Content-Type: header's main type is either
multipart, or there is no Content-Type:
header. In any other situation,
MultipartConversionError is raised.